tr6 mixture

[harveyp1]

I have recently fitted a 5 thou shim into the metering unit as per the article in technical tips by Clive Dempsey, how nice to see a straw coloured spark plug in place of a sooty one, however only onr problem , the exhaust now pops on overrun. Have I gone too lean or is it not cutting mixture back too slowly.

Anybody have any comments please

[Guest P Cobbold]

That 'popping' on the over-run was a distinctive TR6 PI sound. I well remember it - mine ran like that for well over 100,000miles. So I reckon that shim has helped restore the mixture towards normal. Try checking a plug from no 1 or 2 cylinders- mine always ran weak, the rear ones rich. You dont want to run any cylinder too weak.

[JohnC]

I'd be very careful of the exhaust colour + shim approach. With a 5 thou shim you have reduced the fuel delivery by about 3.5cc per 1000 injections. At full throttle this is about 8% (of ~42cc) which may be OK if your metering unit was a bit rich in the first place. BUT you have also reduced fuel delivery by the same amount (not %) at all other throttle openings. At 4"Hg vacuum (motorway cruise-ish) the standard fuel curve dictates about 20cc per 1000 injections, so you're reducing the delivery by 17% or so (assuming your MU was somewhere in the right field to start with). That could take you quite lean. I'm not surprised you're getting popping on over-run, as the standard curve looks for only 1.5-3.0cc per 1000 injections - you're probably at or near zero.

 

It may be that your car was running over-rich to start with, and that the shim has brought at least part of the fuel curve close to where it should be. Unfortunately, you really have no way of knowing, and you are risking doing damage if your guess is wrong. If you want to get it sorted, I suggest you need to either hand over your car to an expert, or to really dive in and make (careful, documented, reversible) adjustments on a rolling road.

 

FWIW, I had my car on a dyno yesterday, tweaking the max fuel using exactly the shims you describe. My exhaust is black, but the AFR at full throttle and 5000rpm is about 12.5:1. When I reduced the fuel using a 2 thou shim, I got the AFR to 13.1:1, but power was down by a couple of hp. A 5 thou shim would have put me the wrong side of stoichiometric. My conclusion is that I need to concentrate on part-throttle metering - max fuel is fine. Not finished yet, but my point is that exhaust colour does not give you a definitive answer. A plug chop is much better, and a dyno better still.

 

Cheers,

John

[Andy Moltu]

The colour of the exhaust isn't a good enough guide to set the mixture by. It was a better guide in the days of leaded fuel but still way too hit & miss.

 

Weakening the top end fueling by 5 thou would only be valid if it was overfueling on full throttle something that can only really be determined by using CO meter or lambda sensor which effectively means using a rolling road. Tickover CO readings & fast idle readings as used in the MOT test are not indicative of what's going on at full throttle under load.

Assuming the metering unit isn't knackered and that Triumph/Lucas knew what fuel settings were appropriate for a standard car then you may well have weakened your mixture excessively.

 

Before weakening the mixture you should consider what else could have been causing your mixture to be rich or appear rich. Vacuum leaks, incorrectly balanced butterflies or tappets all contribute. Similarly ignition faults may also play a part.

 

Popping on overrun is quite common in PI 6's - lean mixture on overrun can worsen this as can air leaks into the exhaust. Some after market cams also do more popping.

 

As John said the only practical way to check the fueling is on a rolling road.